And Samuel never beheld Saul again until the day of his death; for Samuel mourned for Saul; and the LORD repented that He had made Saul king over Israel. {P}

This verse is ambiguous. Either it could mean:

A) Samuel never beheld Saul again till the day of his death (meaning, Samuel died without seeing Saul again)

B) Samuel never beheld Saul again till Saul's day of death which is supported because of the Witch of En-Dor story where Saul sees Samuel again on the eve of battle. According to Jewish law, the night is the beginning of the day, and hence Samuel seeing Saul that night and telling him that tomorrow, Saul and his three sons would be with him was the equivalent of seeing Saul on the day of his death.

Your friend is incorrect. The witch herself sees Samuel (see verse at I Samuel 28:12) as stated by the narrator. Also, the key factor that the man is wearing a me'il, pointing to the precious possession that Chana made for her son, proves it is Samuel.

The question then arises can whiches be trusted to see and speak the truth, or are they among the deceived themselves? A case of the blind leading the blind, so to speak.

Secondly, the Tanach does mention lying siprits in the mouths of "prophets" and a lying spirit which set out to deceive Ahab. If such a spirit has decived the witch by its very appearance, would not its dress be a lesser point?

This depends on what tradition you come from. In the Jewish tradition, there are distinct differences between prophets and witches; the two are very far from being the same. Thus, lying spirits in the mouths of prophets do not equate to an inability to call up the dead. Indeed, King Saul forbade witches from practicing necromancy specifically because they could actually perform it in truth. Had they been charlatans, I doubt he would have minded as much.

Regarding the 'me'il'- no one other than Samuel is described as wearing this particular robe throughout the entire Tanakh. It is his particular characterization, almost a form of synecdoche. He is first introduced as a child in a robe in I Samuel 2:19 and from then on, he is always described as a man wrapped in a robe. Indeed, it is possible his anger over it being torn (which signifies Saul no longer being given charge of the kingdom) may be partially due to the fact that it is the only connection he has to his mother.